Proper control of motor behavior requires finely-tuning of motor neuron excitability. Further, motor neurons are heterogenous and innervate different muscles with diverse functions. However, little is known about regulation of excitability in specific motor neuron subtypes. I propose to compare excitability properties of different zebrafish motor neuron subtypes at different embryonic stages (24 and 48 hpf). Zebrafish primary motor neurons (PMNs) are the first population of motor neurons to control motor behavior, present as early as 17hpf. PMN subtypes differ with respect to soma position, axonal projections, innervated muscle groups and transcription factors expression. However, it is not known whether excitability properties also differ among PMN subtypes. I will take advantage of the Tg(HB9:GFP) zebrafish line that allows identification of PMN subtypes in vivo and perform both current and voltage-clamp experiments to characterize excitability properties. In addition, I will determine whether postsynaptic muscle regulates excitability of specific PMN subtypes in vivo. For these studies, I will take advantage of zebrafish genetic mutants that have well- characterized defects in muscle activity. Because.zebrafish PMN subtypes innervate exclusive and non- overlapping muscle groups in the zebrafish trunk, I will test whether different muscle groups regulate PMN excitability in a subtype specific manner. Overall, these studies will provide new information about in vivo development of motor function in identified motor neuron cell types.